This invention relates to a method of making a twin sheet thermoformed structure and the resultant product of that method.
Single sheet thermoforming has been perceived as lacking in strength for some applications. In an attempt to strengthen single sheet structures, designers have fused the single sheet upon itself to create webbing or cross ribbing. The webbing is generally material that crosses a structure in the article, such as a rib, so as to help stabilize the single sheet structure.
Basic apparatus for performing twin sheet thermoforming is shown by way of example in U.S. Pat. Nos. 3,583,036, 3,787,158, and 3,925,140 to Brown. The method of twin sheet thermoforming is well known in the art to produce hollow structures. In its most basic form, twin sheet thermoforming consists of heating two separate sheets of plastic, forming each sheet to a mold, then fusing the two sheets together by bringing the two molded sheets into contact at selected points where fusion will take place. In one form, the molded sheets are fused together by bringing the respective mold for each sheet together so that the sheets on the surface of the molds contact each other. The intended result is an article having greater strength and/or rigidity than the same article formed from a single sheet of plastic.
A considerable amount of development work in the twin sheet thermoforming art has focused on the engineering of beams or ribs in one or both of the twin sheets to increase the strength of the resultant article for any given amount of thermoformed material. The use of channels (e.g., troughs produced by molding bends in the plastic sheets) or bosses (e.g., circular or trapezoidal impressions formed in one or both sheets) are well known in the art for use as rib structures to strengthen or stiffen the twin sheet thermoformed article. The beam structures are formed by molding the thermoplastic material over the surface of a mold so that the thermoformable sheet conforms to the surface structure of the mold. By way of example, if the surface structure of the mold contains a male rib, thermoforming the thermoformable sheet over the mold surface structure will result in a female rib created in the thermoformable sheet. A rib thermoformed in one sheet may create a bending moment or hinge along its axis. To offset this, a non-parallel rib may be thermoformed in the opposing sheet with the two ribs fused together where they intersect. Another design response has been to reduce the length of particular ribs to reduce the propagation of hinge or bending moment effects. The presence of the ribs may, however, contribute to functional problems not relating to strength, e.g., the ribs may collect water and need to be drained. Ribs running the length of an article may also leave the article prone to warpage, as small expansions or contractions in material may be propagated along the entire article causing it to warp.
The continuing work on designing rib structures for twin sheet thermoformed articles indicates the industry's desire to improve the strength developed per amount of raw material (thermoformable material) used. It is an object of the present invention to provide a twin sheet thermoforming technique which improves the strength or rigidity developed per amount of material used. It is a further object to increase the strength of the resultant articles while reducing hinge or bending moments. It is an object of the present invention to provide a strength-enhancing method of twin sheet thermoforming that allows use of conventional twin sheet thermoformers. It is a further object of the invention to produce articles less susceptible to warping.
The present invention concerns an improvement over either single sheet thermoforming with webbing or cross ribs or conventional twin sheet thermoforming. The present invention provides a method for fusing a single sheet of thermoformable material on itself to create a strength-enhancing rib or beam. The sheet thus fused is then fused to a second sheet in a conventional twin sheet thermoforming operation. In the preferred embodiment of the invention, a mold surface structure for forming a major rib is segregated into discrete elements with spaces between individual elements. In forming the sheet of thermoplastic over the series of discrete elements, a major rib is formed along the line of the elements. Between the individual elements, the thermoplastic material will flow together from either side and when fused together form minor or cross rib elements. A single sheet thus fused is then fused in a conventional twin sheet thermoforming operation to another sheet. The second sheet may be conventionally formed or likewise fused upon itself before being fused to the first sheet. The resultant article has greater strength and rigidity for a given amount of thermoformable material used related to the addition of the minor ribs. The method can be practiced using existing thermoforming machinery to produce a stronger structure on a per weight basis of material used and also has reduced bending moments.
From the subsequent detailed description taken in conjunction with the accompanying drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.